Many techniques have been proposed that relate to flowmeters for measuring the amount of a fluid flowing through a flow channel, such as a circular pipe. In particular, a wide variety of flowmeters using an ultrasonic wave have been proposed because they have many advantages over the other measurement systems, such as that they can measure the flow rate of an opaque fluid, that they do not obstruct the flow because they can perform measurement from the outside through the wall of the circular pipe or the like, and that they can be easily attached to an existing circular pipe.
For example, Japanese Patent Laid-Open No. 58-32121 discloses a phase-difference-based ultrasonic flowmeter that comprises an ultrasonic wave transmitter and an ultrasonic wave receiver disposed at an upstream position and a downstream position in a flow channel to face each other at a predetermined distance and an ultrasonic signal transmitter that controls the ultrasonic wave transmitter to oscillate at a constant frequency (Patent Document 1). The ultrasonic flowmeter according to Patent Document 1 determines the flow rate based on the proportional relationship between the average flow velocity of a fluid and the phase difference between the ultrasonic frequencies measured by the ultrasonic wave transmitter and the ultrasonic wave receiver. The ultrasonic wave transmitter oscillates an ultrasonic wave at a predetermined frequency in response to an ultrasonic signal form the ultrasonic signal transmitter, and the ultrasonic wave receiver measures the frequency of the ultrasonic wave having propagated through the fluid. Then, the phase difference between the frequencies of the ultrasonic waves, and the average flow velocity of the fluid is calculated from the calculated phase difference and corrected with a constant correction coefficient to determine the flow rate.
Furthermore, Japanese Patent Laid-Open No. 2000-97742 discloses a Doppler-type ultrasonic flowmeter that comprises ultrasonic wave transmitting means that emits an ultrasonic wave into a fluid with an ultrasonic transducer, flow velocity distribution measuring means that receives a reflected wave from a reflector and measures the flow velocity distribution of the fluid, and a flow rate calculating means that calculates the flow rate from the flow velocity distribution (Patent Document 2). The Doppler-type ultrasonic flowmeter according to Patent Document 2 calculates the flow rate by the flow velocity distribution measuring means obtaining the flow velocity distribution of the fluid at intervals of about 50 to 100 ms and the flow rate calculating means integrating the flow velocity distribution with respect to the cross-sectional area of the flow channel. In addition, to improve the accuracy of the measurement, it has been proposed to use a plurality of ultrasonic transducers for the flow rate measurement.    Patent Document 1: Japanese Patent Laid-Open No. 58-32121    Patent Document 2: Japanese Patent Laid-Open No. 2000-97742